GETHOSTBYNAME

Section: C Library Functions (3)
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BSD mandoc
BSD 4  

NAME

gethostbyname gethostbyaddr gethostent sethostent endhostent herror - get network host entry  

SYNOPSIS

Fd #include <netdb.h> Ft extern int Fa h_errno ;

Ft struct hostent * Fn gethostbyname char *name Ft struct hostent * Fn gethostbyname2 char *name int af Ft struct hostent * Fn gethostbyaddr char *addr int len, type Ft struct hostent * Fn gethostent Fn sethostent int stayopen Fn endhostent Fn herror char *string  

DESCRIPTION

Fn Gethostbyname , Fn gethostbyname2 , and Fn gethostbyaddr each return a pointer to a Ft hostent structure (see below) describing an internet host referenced by name or by address, as the function names indicate. This structure contains either the information obtained from the name server, named(8), or broken-out fields from a line in /etc/hosts If the local name server is not running, these routines do a lookup in /etc/hosts
struct  hostent {
        char    *h_name;        /* official name of host */
        char    **h_aliases;    /* alias list */
        int     h_addrtype;     /* host address type */
        int     h_length;       /* length of address */
        char    **h_addr_list;  /* list of addresses from name server */
};

#define h_addr  h_addr_list[0]  /* address, for backward compatibility */

The members of this structure are:

h_name
Official name of the host.
h_aliases
A zero-terminated array of alternate names for the host.
h_addrtype
The type of address being returned; usually AF_INET
h_length
The length, in bytes, of the address.
h_addr_list
A zero-terminated array of network addresses for the host. Host addresses are returned in network byte order.
h_addr
The first address in h_addr_list ; this is for backward compatibility.

When using the nameserver, Fn gethostbyname will search for the named host in each parent domain given in the ``search '' directive of resolv.conf5 unless the name contains a dot (``.'' ) If the name contains no dot, and if the environment variable HOSTALIASES contains the name of an alias file, the alias file will first be searched for an alias matching the input name. See hostname(7) for the domain search procedure and the alias file format.

Fn Gethostbyname2 is an evolution of Fn gethostbyname intended to allow lookups in address families other than AF_INET , for example, AF_INET6 . Currently, the Fa af argument must be specified as AF_INET else the function will return NULL after having set Ft h_errno to NETDB_INTERNAL

Fn Sethostent may be used to request the use of a connected TCP socket for queries. If the Fa stayopen flag is non-zero, this sets the option to send all queries to the name server using TCP and to retain the connection after each call to Fn gethostbyname or Fn gethostbyaddr . Otherwise, queries are performed using UDP datagrams.

Fn Endhostent closes the TCP connection.  

ENVIRONMENT

HOSTALIASES
Name of file containing (host alias , full hostname ) pairs.

 

FILES

/etc/hosts
See hosts(5).

 

DIAGNOSTICS

Error return status from Fn gethostbyname and Fn gethostbyaddr is indicated by return of a null pointer. The external integer Ft h_errno may then be checked to see whether this is a temporary failure or an invalid or unknown host. The routine Fn herror can be used to print an error message describing the failure. If its argument Fa string is non-NULL, it is printed, followed by a colon and a space. The error message is printed with a trailing newline.

Ft h_errno can have the following values:

NETDB_INTERNAL
This indicates an internal error in the library, unrelated to the network or name service. Ft errno will be valid in this case; see perror(3).
HOST_NOT_FOUND
No such host is known.
TRY_AGAIN
This is usually a temporary error and means that the local server did not receive a response from an authoritative server. A retry at some later time may succeed.
NO_RECOVERY
Some unexpected server failure was encountered. This is a non-recoverable error, as one might expect.
NO_DATA
The requested name is valid but does not have an IP address; this is not a temporary error. This means that the name is known to the name server but there is no address associated with this name. Another type of request to the name server using this domain name will result in an answer; for example, a mail-forwarder may be registered for this domain.

 

SEE ALSO

hosts(5), hostname(7), named(8), resolver(3), resolver(5).  

CAVEAT

Fn Gethostent is defined, and Fn sethostent and Fn endhostent are redefined, when libc is built to use only the routines to lookup in /etc/hosts and not the name server:

Fn Gethostent reads the next line of /etc/hosts opening the file if necessary.

Fn Sethostent is redefined to open and rewind the file. If the Fa stayopen argument is non-zero, the hosts data base will not be closed after each call to Fn gethostbyname or Fn gethostbyaddr .

Fn Endhostent is redefined to close the file.

 

BUGS

All information is contained in a static area so it must be copied if it is to be saved. Only the Internet address format is currently understood.


 

Index

NAME
SYNOPSIS
DESCRIPTION
ENVIRONMENT
FILES
DIAGNOSTICS
SEE ALSO
CAVEAT
BUGS

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